The trait `radicle_ssh::agent::client::ClientStream` trait was
simplified, because `winpipe::WinStream` is so similar to
`std::os::unix::net::UnixStream` that we can just use the same
implementation for both platforms.
However, obtaining the process identifier of the SSH agent process is
not common on Windows, so we instead track the path that we used to
connect to the agent. This is also helpful on Unix. In some cases, the
path will even contain the process identifier. The path is printed by
`rad self`.
Errors are cleaned up, and error messages are improved.
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